Support DD_APM_TRACING_ENABLED=false

Author: cataphract

CMakeLists.txt

@@ -7,7 +7,7 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 cmake_policy(SET CMP0068 NEW)
 cmake_policy(SET CMP0135 NEW)
 
-set(NGINX_DATADOG_VERSION 1.6.1)
+set(NGINX_DATADOG_VERSION 1.7.0)
 project(ngx_http_datadog_module VERSION ${NGINX_DATADOG_VERSION})
 
 option(NGINX_DATADOG_ASM_ENABLED "Build with libddwaf" ON)

dd-trace-cpp

@@ -1,13 +1,24 @@
 #include "headers.h"
 
+#include <memory>
+
 #include "string_util.h"
 
-namespace datadog::common {
+extern "C" {
+#include <ngx_hash.h>
+}
 
-ngx_table_elt_t *search_header(ngx_list_t &headers, std::string_view key) {
-  ngx_list_part_t *part = &headers.part;
-  auto *h = static_cast<ngx_table_elt_t *>(part->elts);
+namespace {
+
+template <bool delete_header>
+auto search_header_impl(ngx_list_t &headers, std::string_view key) {
+  auto key_lc = std::unique_ptr<u_char[]>{new u_char[key.size()]};
+  std::transform(key.begin(), key.end(), key_lc.get(),
+                 datadog::nginx::to_lower);
+  ngx_uint_t key_hash = ngx_hash_key(key_lc.get(), key.size());
 
+  ngx_list_part_t *part = &headers.part;
+  ngx_table_elt_t *h = static_cast<ngx_table_elt_t *>(part->elts);
   for (std::size_t i = 0;; i++) {
     if (i >= part->nelts) {
       if (part->next == nullptr) {
@@ -19,19 +30,42 @@ ngx_table_elt_t *search_header(ngx_list_t &headers, std::string_view key) {
       i = 0;
     }
 
-    if (key.size() != h[i].key.len ||
-        ngx_strncasecmp((u_char *)key.data(), h[i].key.data, key.size()) != 0) {
+    if (h[i].hash != key_hash || key.size() != h[i].key.len ||
+        memcmp(key_lc.get(), h[i].lowcase_key, key.size()) != 0) {
       continue;
     }
 
-    return &h[i];
+    if constexpr (delete_header) {
+      part->nelts--;
+      if (i < part->nelts) {
+        memmove(&h[i], &h[i + 1], (part->nelts - i) * sizeof(*h));
+      }
+      return true;
+    } else {
+      return &h[i];
+    }
+  }
+
+  if constexpr (delete_header) {
+    return false;
+  } else {
+    return static_cast<ngx_table_elt_t *>(nullptr);
   }
+}
+}  // namespace
+
+namespace datadog::common {
 
-  return nullptr;
+ngx_table_elt_t *search_req_header(ngx_list_t &headers, std::string_view key) {
+  return search_header_impl<false>(headers, key);
 }
 
-bool add_header(ngx_pool_t &pool, ngx_list_t &headers, std::string_view key,
-                std::string_view value) {
+bool delete_req_header(ngx_list_t &headers, std::string_view key) {
+  return search_header_impl<true>(headers, key);
+}
+
+bool add_req_header(ngx_pool_t &pool, ngx_list_t &headers, std::string_view key,
+                    std::string_view value) {
   if (headers.last == nullptr) {
     // Certainly a bad request (4xx). No need to add HTTP headers.
     return false;
@@ -44,13 +78,6 @@ bool add_header(ngx_pool_t &pool, ngx_list_t &headers, std::string_view key,
 
   const auto key_size = key.size();
 
-  // This trick tells ngx_http_header_module to reflect the header value
-  // in the actual response. Otherwise the header will be ignored and client
-  // will never see it. To date the value must be just non zero.
-  // Source:
-  // <https://web.archive.org/web/20240409072840/https://www.nginx.com/resources/wiki/start/topics/examples/headers_management/>
-  h->hash = 1;
-
   // HTTP proxy module expects the header to has a lowercased key value
   // Instead of allocating twice the same key, `h->key` and `h->lowcase_key`
   // use the same data.
@@ -61,6 +88,11 @@ bool add_header(ngx_pool_t &pool, ngx_list_t &headers, std::string_view key,
   }
   h->lowcase_key = h->key.data;
 
+  // In request headers, the hash should be calculated from the lowercase key.
+  // See ngx_http_parse_header_line in ngx_http_parse.c
+  // Response headers OTOH use either 1 or 0, with 0 meaning "skip this header".
+  h->hash = ngx_hash_key(h->lowcase_key, key.size());
+
   h->value = nginx::to_ngx_str(&pool, value);
   return true;
 }

src/common/headers.cpp

@@ -9,7 +9,8 @@ extern "C" {
 
 namespace datadog::common {
 
-/// Searches through an NGINX header list to find a header with a matching key.
+/// Searches through an NGINX request header list to find a header with a
+/// matching key.
 ///
 /// @param headers
 ///     A reference to an NGINX-style list (`ngx_list_t`) containing
@@ -23,9 +24,25 @@ namespace datadog::common {
 ///     A pointer to the matching `ngx_table_elt_t` header element if found,
 ///     or `nullptr` if no header with the given key exists in the list.
 ////
-ngx_table_elt_t *search_header(ngx_list_t &headers, std::string_view key);
+ngx_table_elt_t *search_req_header(ngx_list_t &headers, std::string_view key);
 
-/// Adds a new HTTP header to an NGINX-style header list.
+/// Deletes a request header with the specified key from a NGINX-request header
+/// list.
+///
+/// @param headers
+///     A reference to an NGINX-style list (`ngx_list_t`) containing
+///     `ngx_table_elt_t` elements, typically representing HTTP headers.
+///
+/// @param key
+///     A string view representing the name of the header to delete.
+///     The comparison is case-insensitive.
+///
+/// @return
+///     `true` if a header with the given key was found and deleted;
+///     `false` if no header with the given key exists in the list.
+bool delete_req_header(ngx_list_t &headers, std::string_view key);
+
+/// Adds a new HTTP request header to an NGINX-style header list.
 ///
 /// @param pool
 ///     A reference to the NGINX memory pool (`ngx_pool_t`) used for allocating
@@ -47,7 +64,7 @@ ngx_table_elt_t *search_header(ngx_list_t &headers, std::string_view key);
 /// @return
 ///     `true` if the header was successfully added to the list;
 ///     `false` if memory allocation failed or the list could not be updated.
-bool add_header(ngx_pool_t &pool, ngx_list_t &headers, std::string_view key,
-                std::string_view value);
+bool add_req_header(ngx_pool_t &pool, ngx_list_t &headers, std::string_view key,
+                    std::string_view value);
 
 }  // namespace datadog::common

src/common/headers.h

@@ -57,6 +57,10 @@ struct sampling_rule_t {
 };
 
 struct datadog_main_conf_t {
+  // DD_APM_TRACING_ENABLED
+  // Whether we discard almost all traces not setting  _dd.p.ts
+  ngx_flag_t apm_tracing_enabled{NGX_CONF_UNSET};
+
   std::unordered_map<std::string, ngx_http_complex_value_t *> tags;
   // `are_propagation_styles_locked` is whether the tracer's propagation styles
   // have been set, either by an explicit `datadog_propagation_styles`

src/datadog_conf.h

@@ -22,7 +22,8 @@ DatadogContext::DatadogContext(ngx_http_request_t *request,
                                datadog_loc_conf_t *loc_conf)
 #ifdef WITH_WAF
     : sec_ctx_{security::Context::maybe_create(
-          *loc_conf, security::Library::max_saved_output_data())}
+          security::Library::max_saved_output_data(),
+          security::Library::apm_tracing_enabled())}
 #endif
 {
   if (loc_conf->enable_tracing) {

src/datadog_context.cpp

@@ -7,6 +7,7 @@ extern "C" {
 #include <ngx_http.h>
 }
 
+#include <array>
 #include <string_view>
 
 #include "string_util.h"

src/datadog_directive.h

@@ -24,12 +24,17 @@ class NgxHeaderWriter : public datadog::tracing::DictWriter {
       : request_(request), pool_(request_->pool) {}
 
   void set(std::string_view key, std::string_view value) override {
+    if (value.empty()) {
+      common::delete_req_header(request_->headers_in.headers, key);
+      return;
+    }
+
     ngx_table_elt_t *h =
-        common::search_header(request_->headers_in.headers, key);
+        common::search_req_header(request_->headers_in.headers, key);
     if (h != nullptr) {
       h->value = to_ngx_str(pool_, value);
     } else {
-      common::add_header(*pool_, request_->headers_in.headers, key, value);
+      common::add_req_header(*pool_, request_->headers_in.headers, key, value);
     }
   }
 };

src/ngx_header_writer.h

@@ -3,14 +3,14 @@
 #include <cassert>
 #include <cstdlib>
 #include <exception>
-#include <iterator>
 #include <memory>
-#include <new>
 #include <string_view>
 #include <utility>
 
+#include "datadog/injection_options.h"
 #include "datadog_conf.h"
 #include "datadog_conf_handler.h"
+#include "datadog_context.h"
 #include "datadog_directive.h"
 #include "datadog_handler.h"
 #include "datadog_variable.h"
@@ -28,6 +28,7 @@
 #include "rum/config.h"
 #endif
 #include "common/variable.h"
+#include "ngx_header_writer.h"
 #include "string_util.h"
 #include "tracing_library.h"
 #include "version.h"
@@ -278,6 +279,8 @@ static ngx_int_t datadog_master_process_post_config(
   return NGX_OK;
 }
 
+static ngx_int_t on_precontent_phase(ngx_http_request_t *request) noexcept;
+
 static ngx_int_t datadog_module_init(ngx_conf_t *cf) noexcept {
   ngx_http_next_header_filter = ngx_http_top_header_filter;
   ngx_http_top_header_filter = on_header_filter;
@@ -313,6 +316,11 @@ static ngx_int_t datadog_module_init(ngx_conf_t *cf) noexcept {
   }
 #endif
 
+  if (set_handler(cf->log, core_main_config, NGX_HTTP_PRECONTENT_PHASE,
+                  on_precontent_phase) != NGX_OK) {
+    return NGX_ERROR;
+  }
+
   // Add default span tags.
   const auto tags = TracingLibrary::default_tags();
   if (!tags.empty()) {
@@ -493,3 +501,31 @@ static char *merge_datadog_loc_conf(ngx_conf_t *cf, void *parent,
 
   return NGX_CONF_OK;
 }
+
+static ngx_int_t on_precontent_phase(ngx_http_request_t *request) noexcept {
+  auto *ctx = get_datadog_context(request);
+  if (!ctx) {
+    return NGX_DECLINED;
+  }
+
+  // inject headers in the precontent phase into the request headers
+  // These headers will be copied by ngx_http_proxy_create_request on the
+  // content phase into the outgoing request headers (probably)
+  RequestTracing &tracing = ctx->single_trace();
+  dd::Span &span = tracing.active_span();
+  span.set_tag("span.kind", "client");
+
+  datadog::tracing::InjectionOptions opts{};
+#ifdef WITH_WAF
+  if (auto sec_ctx = ctx->get_security_context()) {
+    if (sec_ctx->has_matches()) {
+      opts.trace_source = {'0', '2'};
+    }
+  }
+#endif
+
+  NgxHeaderWriter writer(request);
+  span.inject(writer, opts);
+
+  return NGX_DECLINED;
+}

src/ngx_http_datadog_module.cpp

@@ -240,12 +240,6 @@ RequestTracing::RequestTracing(ngx_http_request_t *request,
   // We care about sampling rules for the request span only, because it's the
   // only span that could be the root span.
   set_sample_rate_tag(request_, loc_conf_, *request_span_);
-
-  // Inject the active span
-  NgxHeaderWriter writer(request_);
-  auto &span = active_span();
-  span.set_tag("span.kind", "client");
-  span.inject(writer);
 }
 
 void RequestTracing::on_change_block(ngx_http_core_loc_conf_t *core_loc_conf,
@@ -275,12 +269,6 @@ void RequestTracing::on_change_block(ngx_http_core_loc_conf_t *core_loc_conf,
   // We care about sampling rules for the request span only, because it's the
   // only span that could be the root span.
   set_sample_rate_tag(request_, loc_conf_, *request_span_);
-
-  // Inject the active span
-  NgxHeaderWriter writer(request_);
-  auto &span = active_span();
-  span.set_tag("span.kind", "client");
-  span.inject(writer);
 }
 
 dd::Span &RequestTracing::active_span() {